SLG's Role in Self-Incompatibility

Three proteins encoded by the BrassicaS-locus, S-receptor kinase (SRK), S-locus protein 11 (SP11), and S-locus glycoprotein (SLG), are involved in preventing self-pollination through a process known as self-incompatibility. SP11 is found in the pollen coat, whereas SRK and SLG are present on the stigma, the part of flowering plants that binds pollen. SP11 is thought to be the ligand of SRK, but the function of SLG remains unknown. Takayama et al. have solved how the three proteins cooperate. 125I-labeled SP11 bound to SRK, and the presence of SLG in this protein complex increased the affinity of SRK for SP11. When microsomal membrane preparations from plants were treated with nonionic detergents rather than with more disruptive ionic detergents, the association of SRK and SLG was preserved, indicating that previous unsuccessful attempts to show an association between SRK and SLG may have failed because of the inability to preserve their association, rather than indicating a lack of association. Autophosphorylation of SRK was SP11-dependent, suggesting that self-incompatibility arises from intracellular signals propagated by SRK. Thus, SLG appears to have an important role in the formation of signaling complexes that proscribe self-incompatibility.